Method and apparatus for congestion control for packet-based networks using call gapping and rerouting in the peripheral domain

ABSTRACT

The invention pertains to methods and apparatus for determining when and how to perform communications network congestion control tactics with respect to packet-based networks that are used to interconnect other networks, particularly time division multiplexed (TDM) networks. The congestion control mechanisms include rerouting and call gapping in the TDM networks to reduce congestion at ports of the packet-based network.

FIELD OF INVENTION

The invention pertains to congestion control for communications networksand, particularly, packet-based voice communications networks.

BACKGROUND OF THE INVENTION

The use of packet-based networks for communicating data between variouslocations is well known. Packet-based networks also are increasinglybeing used to carry voice or combined voice and data traffic.Particularly, public telephone companies are using packet-based networksas part of the public telephone networks. The two types of packet-basednetworks in most common use today are ATM (Asynchronous Transfer Mode)networks and IP (Internet Protocol) networks. The small but growingnumber of networks which use either of these types of packet-basednetworks for transmitting voice (or voice and data) are termed VTOA(Voice Traffic Over ATM) and VoIP (Voice-over-IP) networks,respectively.

In packet-based networks, data streams transferred between two nodes ofthe network are transferred in discrete packets. Packets may be ofconsistent size or may be variably sized. Each packet includes a fieldof data which may be preceded and/or followed by non-data informationsuch as preamble and housekeeping information (such as data source anddestination addresses and the like). The data source destination fieldsof the packet are used by the switches and routers in the network toroute the data through network nodes from source node to destinationnode.

The destination node reconstructs the data in the proper order based onthe housekeeping information to accurately recreate the transmitted datastream. Accordingly, a continuous stream of data can be generated at asource node of the network, transmitted in separate packets (which caneven travel separate routes) to the destination node and be reassembledat the receiving node to recreate a continuous stream of receive data.The fact that the data was packetized for transmission and reassembledis transparent to the users at the source and destination nodes.

However, when congestion on the network exceeds the physicalcapabilities of the network, i.e., the amount of data that must betransmitted via the network for all of its source-destination node pairsexceeds the physical capabilities of the network, packets get droppedand the apparently seamless transmission of data falls apart.Particularly, using a voice link as an example, transmission links inwhich packets are being dropped will have noticeably reduced audioquality. The receiving party may perceive noise, delay jitter and evengaps in the received voice transmission. Delay jitter occurs when thepackets do not arrive quickly enough to be recreated seamlessly toproduce a continuous stream of data. Noise and gaps appear when packetsare completely dropped.

FIG. 1 is a block diagram of an exemplary communication network 10,including a packet-based communication sub-network 12. The packet-basedsub-network 12 of FIG. 1 forms a portion of an overall telephonecommunications system 10 in this particular example. The packet-basednetwork 12 interconnects a plurality of public service telephonenetworks (PSTNs) 14. In other words, some nodes of the packet-basednetwork 12 are gateway nodes through which data from other networks, forexample, time division multiplexed (TDM) PSTN networks 14 istransferred. The network nodes through which the PSTNs are coupled tothe packet-based network are termed gateways. The gateways 18 convertvoice data from the protocol of the PSTN networks 14 (e.g., a TDMprotocol) to the protocol of the packet-based network 12 (i.e.,packets).

Using voice data being transmitted from PSTN 14 a to PSTN 14 b as anexample, gateway 18 a converts the TDM voice into packet voice andtransmits it to an edge switch or edge router 20 a. From edge router 20a, the data is directed to edge router 20 b, preferably, directly.However, the data may be routed through one or more switches and/orrouters 24. Edge router 20 b directs the packets to gateway 18 b, wherepacket voice data is converted to the TDM format and then forwarded onthe PSTN network 14 b.

The term edge switch or edge router refers to a device that is primarilyresponsible for performing network access functions for the packet-basednetwork, such as low level traffic classification, aggregation andmanagement. As its name implies, an edge switch or edge router typicallyis coupled directly to a gateway into the network, i.e., it is at theedge of the packet-based network. The network further comprises coreswitches and core routers 24. Both of these terms refer to devices thatperform larger scale traffic classification, aggregation and management.

There are several known mechanisms for handling congestion incommunications networks. For instance, call blocking is a mechanism bywhich new calls (i.e., source/destination node pairs) are not acceptedby the network when the quality of existing calls starts degrading dueto congestion. Another method known for reducing congestion at thecongested point is by rerouting calls in the PSTN networks.Particularly, the TDM networks may be coupled to the centralpacket-based network at multiple gateways. If a path through oneparticular gateway is congested, the PSTN network can reroute calls toanother gateway through which less congested paths in the packet-basednetwork can be accessed.

It is also possible to use compression algorithms to reduce thebandwidth needed for a call and thus allow more calls to beaccommodated. With respect to voice communications, many voicecompression algorithms are known. However, compression typically leadsto loss of information. Generally, the greater the amount ofcompression, the greater the loss of data. In connection with voicecommunications, generally, the greater the amount of compression, thelower the audio quality of the call.

Call gapping in the PSTN networks is another possible mechanism forreducing congestion.

It is an object of the present invention to provide a network congestionmanagement method and apparatus for use in connection with packet-basednetworks.

SUMMARY OF THE INVENTION

The invention is a method and apparatus for congestion management in apacket-based network that interconnects a plurality of peripheralnetworks. We disclose a method and apparatus by which the amount ofcongestion is quantified and particular corrective actions and the levelof those corrective actions necessary to eliminate or reduce data lossis generated. The generated data may be transferred to those networkelements (or the peripheral networks) which can implement the indicatedcorrective actions or may be implemented automatically by a centralizednetwork congestion manager circuit (e.g., a dedicated digital signalprocessor). The system operates essentially in real time. Among thecorrective measures supported by the invention are reconfiguration ofbandwidth pipes, call blocking, rerouting in the peripheral networks(e.g., PSTN networks that are coupled to a central packet-based networkat multiple gateways), selective voice compression on new calls and callgapping in the peripheral network.

We disclose a method and apparatus for identifying a set of virtualpipes in the network that are experiencing call blocking higher than adesignated call blocking threshold and identifying a set of PSTNswitches in the peripheral network whose calls are being carried in thevirtual pipes belonging to that set. The method and apparatus furtherincludes a mechanism for calculating a call gapping rate/ratio and/orrerouting scheme for use in the peripheral network domain andcommunicating that information to the appropriate gateways for executionof the recommended call gapping and/or rerouting regime in theperipheral domain.

The network congestion management in accordance with the presentinvention may be a centralized system or it may be a distributed systemin which multiple nodes of the network contain elements of the networkcongestion management hardware and software.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a telecommunications network including acentral packet-based network interconnecting multiple PSTN networks inaccordance with the prior art.

FIG. 2 is a block diagram of a telecommunications network in accordancewith the present invention including a central packet-based network forinterconnecting multiple PSTN networks.

FIGS. 3A, 3B, and 3C comprise a flow diagram illustrating an embodimentin accordance with the present invention specifically adapted for an ATMnetwork in which the network congestion management method and apparatuscalculates information necessary for resizing virtual trunk groups torelieve congestion, generates proposed actions to relieve congestion andforwards the proposed action information to the network nodes capable ofexecuting the action.

FIGS. 4A, 4B, and 4C comprise a flow diagram illustrating an embodimentof the invention specifically adapted to an ATM network in which thenetwork congestion management method and apparatus calculatesinformation necessary for call gapping and rerouting calls in the PSTNdomain of peripheral networks to relieve congestion, generates proposedactions to implement the call gapping and rerouting and forwards theproposed action information to the network gateway nodes for forwardingto the PSTN networks for execution.

FIGS. 5A and 5B comprise a flow diagram illustrating an embodiment inaccordance with the present invention specifically adapted for an ATMnetwork in which the network congestion management method and apparatusidentifies virtual trunk groups that are experiencing excessivecongestion and calculates information necessary for determining a levelof voice compression desirable to reduce congestion to an acceptablelevel.

FIGS. 6A, 6B, and 6C comprise a flow diagram illustrating an embodimentof the invention specifically adapted to an IP network on which thereal-time protocol is not implemented which identifies gateway pairsexperiencing excessive congestion in the IP network and generates thenecessary call blocking rate as a fraction of arriving calls for theidentified gateway pairs in order to relieve the congestion.

FIGS. 7A and 7B comprise a flow diagram illustrating an embodiment ofthe invention specifically adapted for IP networks on which thereal-time protocol is implemented which identifies gateway pairsexperiencing excessive congestion in the IP network and generates thenecessary call blocking rate as a fraction of arriving calls for theidentified gateway pairs in order to relieve the congestion.

FIGS. 8A, 8B, 8C, and 8D comprise a flow diagram illustrating anembodiment of the invention specifically adapted for IP networks inwhich the network congestion management method and apparatus identifiespacket-voice gateways experiencing excessive congestion, identifiesalternate source packet-voice gateways and implements call gapping andrerouting calls in the PSTN domain of peripheral networks to reducecongestion.

FIGS. 9A, 9B, 9C, and 9D comprise a flow diagram illustrating anembodiment of the invention specifically adapted for IP networks inwhich the network control management method and apparatus identifiescongested packet voice gateway pairs whose blocking rate can be reducedby using higher compression rates and generates the necessarycompression rate to bring congestion below a predetermined level.

DETAILED DESCRIPTION OF THE INVENTION

The network control management method and apparatus of the presentinvention can perform three critical congestion management functions,namely, (1) collection of performance related statistics from networkelement management systems for traffic surveillance and performancemonitoring, (2) identification of potential congestion control actionsto be taken by network managers on target network elements (NEs), and(3) implementation of those congestion control actions. In someembodiments, the network congestion management method and apparatus ofthe present invention may simply calculate and collect relevantinformation for informational purposes. Alternately, it may additionallyforward such information to other network managers for furtherprocessing and implementation of the identified corrective actions. Inother embodiments, some automatic control for implementing said actionscan be included in a centralized network congestion management apparatusmechanism. The types of network elements supported by the presentinvention include ATM edge and core switches, IP edge and core routers(switches) and ATM and IP packet/voice gateways (PVGs).

Below we describe several preferred embodiments of the inventionparticularly adapted for ATM and/or IP networks. However, it should beunderstood that the invention is applicable to other types ofpacket-based networks.

For exemplary purposes and to illustrate highly practical embodiments ofthe invention, we assume that network congestion management capabilitiesfor the exemplary ATM embodiments are consistent with the ATMF UNI 3.1capabilities and, for IP networks, with the RFC-1213 capabilities.

FIG. 2 is a block diagram of a network system in accordance with thepresent invention. This exemplary network is a public telephone networkand therefore has certain predetermined parameters. For instance, eachcall has a bandwidth of 64 Kbps. The network congestion management inaccordance with the present invention is centralized in this embodiment.However, it should be understood that the network congestion managementhardware and software in accordance with the present invention also canbe distributed throughout the network, such as at individual gatewaynodes 18.

The packet-based network 120 interconnects a plurality of public servicetelephone networks (PSTNs). The gateways 18 convert data from theprotocol of the PSTN networks 14 (a TDM protocol) to the protocol of thepacket-based network 120 (i.e., packets).

Using voice data being transmitted from PSTN 14 a to PSTN 14 b as anexample, gateway 18 a converts the TDM voice data into packet voice andtransmits it to an edge switch or edge router 20 a. From edge router 20a, packets are directed to edge router 20 b. The transfer between edgerouter 20 a and edge router 20 b may be direct or packets may be routedthrough one or more core switches and/or routers, such as switch/router24. Edge router 20 b directs packets to gateway 18 b, where packet voiceis converted to the TDM format of PSTN network 14 b and forwarded tothat network.

The network congestion manager (NCM) 38 receives traffic informationfrom the various network elements, including packet gateways, edgeswitches, edge routers, core switches and core routers over controllines 40 and returns traffic control information to them over controllines 42. The NCM may comprise any appropriate type of data processingdevice such as a microprocessor, micro controller, state machine orcombinational logic.

The NCM in accordance with the present invention should be adapted tothe particular network traffic information available from the networkelements. For instance, most ATM networks have a greater amount oftraffic control and information than IP networks.

Below, the invention will first be described in connection with an ATMnetwork having all the network traffic and congestion managementcapabilities in support of the ATMF UNI 4.0 specifications as well asATMF VTOA/AAL2 services. In this embodiment, the congestion controlmechanisms are targeted to the gateway devices. However, it should beunderstood that the invention is equally applicable to any other type ofnetwork element, including the edge and core switches and routers aswell as IP Network Access Servers.

In a preferred ATM embodiment, the network congestion management isbased primarily on the IETF simple network management protocol (SNMPv2)network management framework. The SNMPv2 framework consist of thefollowing components.

-   -   RFC 1902: SMI definition, the mechanisms used for describing and        naming objects for the purpose of management;    -   RFC 1903: Textual conventions for version two of the SNMPv2;    -   RFC 1904: Conformance statements for SNMPv2;    -   RFC 1905: Protocol operations for SNMPv2;    -   RFC 1906: Transport mappings for SNMPv2;    -   RFC 1907: Management information base for SNMPv2; and    -   RFC 1908: Co-existence between SNMPv1 and SNMPv2.        Further, the invention will be described with respect to a        network utilizing standardized MIB (Management Information        dataBase) data. However, it should be understood that the        present invention is adaptable to any MIB protocol.

I. Congestion Indicators

Many packet-based networks, and particularly ATM networks, alreadygenerate a significant amount of information indicative of the amount ofcongestion at or between various network elements. These include callblocking data, transport loss data, transport delay data and transportjitter (delay variability) data.

Call blocking applies to network elements where admission control,either via an explicit admission control function or implicit due toresource contention and blocking, is exercised. Call blocking is used asa congestion management parameter in PSTN TDM networks and ATMpacket-based networks. It is less prevalent with respect to IPpacket-based networks because of the connectionless nature of theservice provided by IP networks and the lack of end-to-endQuality-of-Service management support.

In ATM networks, a threshold for call blocking for a network element,such as a gateway, may be set. If the amount of call blocking occurringat that network element exceeds the threshold, it is typically anindication of insufficient network resources allocated to meet thepresently existing traffic demand at that node.

Transport loss, transport delay and transport jitter apply mostly topacket-based networks rather than TDM networks as a measure ofcongestion.

Other indicators of network congestion include resource utilizationinformation, the number of active calls and the receive buffer overflowor underflow information at gateways. Resource utilization is usuallygiven as a percentage of the total capacity and can be used as animplicit indicator of congestion for networks which do not support amore direct indication of congestion. The number of active calls also isan obvious implicit indicator of congestion at a network element.

Further, both VTOA and VoIP networks utilize re-assembly buffers forreceive data into the network at the packet/voice gateways. Thesebuffers are used to reconstruct the voice stream. Re-assembly bufferoverflow and underflow is an indication of excessive delay jitter andinsufficient re-assembly buffer space, respectively and, thus, also isan indirect measure of congestion at the gateway.

Table 1 below shows the congestion indicator information made availableby various standardized Management Information dataBase (MIB) modules.

MIB Modules IF & RMON ATM ATOM TCP/IP MIBs F MIB MIB (RFC RMO (RFC- (RFC2233/202 RTP Parameter N 1695) 1213) 1) MIB No. of successful X callsetup attempts No. of call setup X attempts Total connection X time No.of DSO X channels No. of transmitted X cells No. of received X cells No.of cell lost X AAL1 Buffer X overflow/underflow No. of transmitted X X XX octets No. of received X X X X octets No. of packet lost X X X DelayJitter X

Congestion at network elements commonly results in decreasedperformance. There are several options for addressing networkcongestion. They include expanding the physical resources of the networksuch as by adding network links, utilizing bigger gateways, adding moreports, etc. Non-hardware options include (1) resizing virtual trunkgroups in ATM networks in order to meet the demand at particular ports,i.e., gateways, (2) using voice compression to reduce the bandwidth ofeach source/destination link, (3) call gapping in the packet-basednetwork, (4) rerouting data through the peripheral networks coupled tothe packet-based network (for packet-based networks which are coupled toother networks through multiple gateways), and (5) call blocking.

II. Congestion Control Mechanisms

Table 2 below is a list of generic parameters which are useful indetecting and determining how to solve network congestion problems.

N: Virtual Trunk Group size in terms of DSO channels λ(t):Non-stationary call attempt or arrival rate μ: Average service time percall ρ(t): Non-stationary offered load B_(N)(t, ρ): Non-stationary callblocking probability B₀: Blocked calls objective n(t) Non-stationarynumber of call attempts T Polling interval

In preferred embodiments, the network congestion management method andapparatus of the present invention monitors various performance relateddata and, as needed, correlates them to detect congestion-related eventsin the network. It then generates information as to solutions to reducethe congestion. The network congestion management method and apparatusof the present invention (hereinafter NCM) can simply calculate andstore congestion information from the available congestion indicatorsfor use by the network administrator who can then manually intervene toperform congestion relief steps (hereinafter termed a level-0embodiment). More preferably, however, the NCM further determinesrecommended corrective actions and provides that information via controllines to the appropriate network elements so that they can take thenecessary actions to reduce congestion (hereinafter termed a level-1embodiment). Alternately, the NCM itself can be further equipped toautomatically execute the recommended actions (hereinafter termed alevel-2 embodiment).

This specification specifically discusses five congestion reliefmechanisms, namely, (1) resizing of virtual trunk groups, (2) callblocking, (3) rerouting of calls in the PSTN domain, (4) voicecompression, and (5) call gapping in the PSTN domain. The resizing ofvirtual trunk groups congestion relief mechanism is most suitable forATM networks as opposed to IP networks, since IP networks do not utilizethe concept of virtual trunk groups. However, it is possible toimplement a similar scheme in IP networks, such that resizing of virtualtrunk groups could also be applied to IP networks. On the other hand,the call blocking congestion relief mechanism is particularly adaptedfor use with IP networks because of the general absence of use of theconcept of virtual pipelines (or trunk groups) in IP networks. The threeother congestion relief mechanisms, however, are equally applicable toATM and IP networks as well as other types of packet-based networks.

III. ATM Congestion Management Embodiments

A. Resizing of Virtual Trunk Groups

FIG. 3 is a flow diagram illustrating an embodiment of the presentinvention specifically adapted for, yet not limited to, ATM networks inwhich network congestion is managed by real-time resizing of virtualtrunk groups. FIG. 3 comprises portions 3A, 3B and 3C and illustrates alevel-2 embodiment in which the NCM monitors the available performancerelated information to detect congested virtual trunk groups (VTGs) inthe network, calculates the peak cell rate (PCR) needed of a PVG inorder to reduce cell blocking to a desired rate and determines availablepaths for resizing VTGs.

The relevant data which are readily available in ATM networks and isused herein include (1) the fraction of blocked calls per virtual trunkgroup (b), (2) the call arrival rate per VTG (λ), (3) the bandwidth percall (e.g., 64 Kbps for a telephone network), (4) the number of channelsper virtual trunk group (N), (5) the blocking threshold (T), (6) theaverage holding time per call, (1/μ) and (7) network connectivity andtopological information.

T is a predetermined number which is set by the network administratorand represents the amount of call blocking that the system administratordeems tolerable before congestion management measures are to be taken.It can conceivably be set to 0, if desired, but usually some callblocking will be deemed acceptable in order to reduce system hardwarerequirements, some of which would be needed only during rare peak usagetimes.

Referring to FIG. 3, variable definitions are given in block 300. Instep 301, variable i is set to 1. Variable i will be used to designatethe various VTGs of the network. In step 303, a VTG corresponding to iis selected. In step 305, a variable M_(i) is set to the actual size,N_(i), of the selected VTG. The size is designated in terms of thenumber of DSO channels in the VTG. In step 307, it is determined whetherthe fraction of blocked calls for VTG_(i) exceeds the threshold, i.e.,if b_(i)>T. If the current VTG is experiencing less call blocking thanthe threshold level, then it is possible that some of the bandwidth fromthat VTG can be reassigned to another VTG that is exceeding the callblocking threshold. Accordingly, if b_(i)<T, flow proceeds to steps 308through 327 to resize this VTG, if possible.

In steps 308 through 315, the necessary channel capacity of the VTG iscalculated. Specifically, in step 308, the VTG size is reduced by oneDSO channel for purposes of calculating whether the VTG can be reducedin size and still meet the call blocking parameter. Note that the VTG isresized for calculation purposes only, it is not actually resized untilstep 320, as will be described below. Given the call arrival rate(λ_(i)) at VTG_(i), the average holding time (1/μ) and the number ofavailable channels (N_(i)), call blocking numbers can be computed usingthe well known Erlang blocking formula for a stationary queuing system.In step 309, the channel capacity for a VTG of size M_(i) is calculated.The call blocking rate may be calculated as follows: $\begin{matrix}{{B_{M_{i}}\left( {\lambda_{i}/\mu} \right)} = {\frac{\left( {\lambda/\mu} \right)^{M_{i}}/M_{i}}{\sum\limits_{n = 0}^{n = {Mi}}{\left( {\lambda/\mu} \right)^{n}/{n!}}} = \frac{\rho\; B_{N}\rho}{N + 1 + {\rho\;{B_{N}(\rho)}}}}} & \left( {{{Eq}.\mspace{14mu} 1}a} \right)\end{matrix}$Alternately, it may be represented as: $\begin{matrix}{{B_{N - 1}(\rho)} = \frac{{NB}_{N}(\rho)}{\rho\left\lbrack {1 - {\rho\;{B_{N}(\rho)}}} \right\rbrack}} & \left( {{{Eq}.\mspace{14mu} 1}b} \right)\end{matrix}$

For a non-stationary system, which is the case of particular interest,one can apply equation 1a or 1b with a non-stationary offered load givenby: $\begin{matrix}{{\rho(t)} = {{{\lambda(t)}/{\mu(t)}}\mspace{14mu}{or}}} & {{Eq}.\mspace{14mu}\left( {2a} \right)} \\{{\rho(t)} = {\left\lbrack {{\lambda(t)} - {\rho(t)}} \right\rbrack/{\mu(t)}}} & {{Eq}.\mspace{14mu}\left( {2b} \right)}\end{matrix}$

Equation 2a is usually referred to as a point-wise stationaryapproximation (PSA), while equation 2b is referred to as a modifiedoffered load (MOL) approximation. The two solutions for ρ(t) areequivalent when the system is stationary, that is, when the call arrivalprocess is independent of time.

When λ(t) is well-known, i.e., when the arrival process is completelyknown as a function of time, the PSA approximation tends to overestimatethe blocking probability while the MOL approximation tends tounderestimate the blocking probability. In practice, the arrival processis not known ahead of time.

Typically, λ(t) would be estimated from the accumulated number of callattempts over a time interval of length T as: $\begin{matrix}{{\lambda\left( {t + T} \right)} = {\left\lbrack {{n\left( {t + T} \right)} - {n(t)}} \right\rbrack/T}} & {{Eq}.\mspace{14mu}(3)}\end{matrix}$

Equation (3) tends to bias the actual call attempt rate at xt because itis an averaging operation. For models such as MOL models, the situationis more complex given that typical parameter estimation withdifferential operators tends to procure noisy and less stableestimators. These kinds of estimators typically are biased, and,depending on the operating regime, can overestimate or underestimate theparameter of interest. In particular, the estimator for the call attemptrate λ(t) in equation (3) underestimates the instantaneous call attemptrate when the call rate is increasing, but it overestimates the callrate when the call rate is decreasing. Thus, the behavior of the systemwhen all parameters are known ahead of time can be substantiallydifferent from the sample behavior. Depending on the particularcircumstances, one might choose to use equations (1a) and (2a) orequations (1b) and (2b).

After the non-stationary call blocking probability for this VTG iscalculated in step 309, the calculated call blocking probability iscompared with the call blocking threshold in step 311. If it is greaterthan the call blocking threshold, this means that the VTG may be able tobe further reduced in size and the extra bandwidth that is presentlyassigned to it can be reassigned to a congested VTG. Accordingly, flowwould loop back to step 308 where the interim VTG size, M_(i), isdecremented by one more DSO channel. The call blocking probability isthen recalculated in step 309 with the smaller value of M_(i). Steps308, 309, and 311 are repeated until the call blocking probability isgreater than the threshold. At that point, flow proceeds to step 315where the value of M_(i) is incremented by one to return to a VTG sizethat keeps the call blocking probability below the threshold.

Then flow proceeds to step 317 where a peak cell rate (PCR)corresponding to channel capacity M_(i) is computed.

Peak cell rate can be determined readily. Given the number of calls thatneed to be carried, the peak cell rate for structured N×64 basic servicefor VTOA networks can be computed as: $\begin{matrix}{{PCR} = {\frac{\left( {8000 \times N} \right)}{46.875}\mspace{14mu}{cells}\mspace{14mu}{per}\mspace{14mu}{second}}} & {{Eq}.\mspace{14mu}(4)}\end{matrix}$However, peak cell rate formulation for other types of services such asunstructured BS1/E1/DS3/E3, DS1/E1 with CAS can be found in ATM formsCES Interoperability Specification version 2.0 (af-vtoa-oo78.ooo). Sincethe notion of virtual trunk group does not exist in current VoIPspecifications, there is no standard conversion rule for peak cell ratefor IP networks.

In steps 319–327, the underutilized VTGs are reduced in size to theextent possible to free up bandwidth for use in increasing the size ofover-utilized VTGs. In step 319, it is inquired whether the peak cellrate of this VTG can be renegotiated (as determined in steps 308–315).If so, flow proceeds to step 320 where the allocated bandwidth to theVTG is reduced to accommodate M_(i) calls. If not, then flow proceeds tostep 321 where it is determined if a path that can accommodate the VTGwith the newly calculated PCR exists. The software uses the networkconnectivity and topological information to determine if there are anypaths that can accommodate a VTG with the bandwidth corresponding to thenewly determined peak cell rate. If so, flow proceeds to step 323 wherea new VTG with the new PCR is created. As shown in step 325, all newcalls will be directed to the new VTG_(i). As shown in step 327, the oldVTG_(i) will be deleted when no calls are using it any longer.

From either step 320 or step 327, flow proceeds to step 329. In step329, it is determined if all of the VTGs in the network have beenchecked. If not, variable i is incremented (step 331) and flow proceedsback to step 303 for operation on the next VTG. For every VTGexperiencing call blocking less than the threshold, flow again proceedsthrough steps 309 through 327 to reduce the size of the VTG, ifpossible.

For every VTG which is experiencing call blocking exceeding the callblocking threshold, flow will instead proceed from step 307 to steps 333through 340. Particularly, step 333 is essentially identical to step309, in which the non-stationary call blocking probability for a VTG ofsize M_(i) is calculated. In step 335, it is determined if thecalculated probability is less than the predetermined call blockingthreshold.

If not, flow proceeds to step 337 where M_(i) is incremented by 1 andthe call blocking probability is re-calculated for the new value ofM_(i). As can be seen, flow proceeds through steps 333 through 337 untila value for M_(i) (the size of the VTG in terms of DSO channels), isreached such that the calculated call blocking probability is less thanthe call blocking threshold. When that size is determined, flow proceedsfrom decision step 335 to step 339. In step 339, a value D_(i) isdetermined which is the difference between the present size, N_(i), ofthat VTG and the just calculated VTG size, M_(i), needed to bring thecall blocking probability number below the predetermined call blockingthreshold.

In step 340, the VTG is inserted into an ordered list, L, of VTGs thatare exceeding the call blocking threshold. The list is ordered such thatthe VTG corresponding to the highest value for D_(i) is listed first andthe VTG corresponding to the lowest value of D_(i) is listed last. Flowthen proceeds to step 329 to determine if all VTGs have been checked. Ifnot, i is incremented (step 331) and flow returns to step 303 forprocessing of the next VTG.

When all VTGs have been checked, flow proceeds to decision step 341. Ifno VTGs made it onto list L, (i.e., if no VTGs are overly congested),then the operation ends since no congestion relief operations arenecessary. However, if there is at least one VTG in list L, flowproceeds to step 343.

In step 343, the first VTG is removed from list L. In step 345, a peakcell rate for the desired size of that VTG is calculated using equation(4). In step 347, it is inquired if the PCR of a VTG can bere-negotiated in this network. If so, flow proceeds to step 348 wherethe bandwidth of the VTG is increased. Then flow returns to step 341 tomove on to the next VTG in list L, if any.

If, on the other hand, the network cannot re-negotiate VTG size, flowproceeds to step 349. In step 349, it is determined if a path existsthat can accommodate a new VTG having M_(i) DSO channels. If so, flowproceeds to step 357 where such a VTG is created. As shown in step 359,all new calls are then directed to the new VTG. Then, as shown in step361, when no calls are using the old VTG_(i) any longer, the old VTG_(i)is deleted. Flow then proceeds back to step 341 so that the next VTG inlist L, if any, can be operated upon.

If, however, in step 349, there is no such path, flow proceeds to step351. In step 351, a peak cell rate is calculated for a number of DSOchannels, D_(i), where D_(i) is the difference between the actual numberof channels in VTG_(i) and the number of channels for VTG_(i) needed toreduce the call blocking probability below the predetermined callblocking threshold. (D_(i) was determined in step 339).

Flow then proceeds to step 353 where, if possible, a new VTG of a sizethat can accommodate D_(i) channels is created. Flow then proceeds tostep 355 where $\frac{D_{i}}{M_{i}}$of the calls from the old VTG are directed to the new VTG created instep 353. Flow then proceeds back to step 341 in order to process thenext VTG, if any, in list L.

The flow chart of FIG. 3 describes a level-2 implementation of thepresent invention. It should be understood by those skilled in the artthat a level 0 implementation would include only steps 301, 303, 305,307, 333, 335, 339, 340, 329, 341, 343 and 345. In step 335, flow wouldproceed back to step 331. An affirmative response in step 307 wouldcause flow to proceed back to step 331.

Further, those of skill in the art also will understand that a level-1implementation in accordance with the present invention would beessentially the same as described in connection with the level-2embodiment of FIG. 3, but without the implementation steps, such assteps 320, 323, 325, 327, 353, 355, 357, 359 and 361.

The following implementation considerations should be noted. As notedabove, the point-wise stationary approximation (PSA) model for theoffered load utilized in equations 1a and 2 a shows a strong correlationto the stochastic behavior of the estimated call attempt rate. Thus, astraightforward implementation of the PSA model would result inunder-estimation of the required VTG size. Also as noted above, thediscreet implementation of the modified offered load (MOL) modelutilized in equation 2b (hereinafter termed D-MOL) would be${\rho(t)} = {{\frac{\lambda(t)}{\mu} \times \mu\; T} + {{\rho\left( {t - T} \right)} \times \left\lbrack {1 - {\mu\; T}} \right\rbrack}}$Yet, being a discrete approximation of the exact MOL model, suchdiscrete steps can produce unstable estimators over certain operationalregimes, particularly with decreasing call rates over time. A simple fixis to use the D-MOL model to estimate the call attempt rate when callrate has been historically increasing over time and use the PSA modelwhen call rate has been historically decreasing over recent time. Thus,${\rho\left( {t + T} \right)} = {\frac{\lambda(t)}{\mu}\mspace{14mu}{PSAModel}}$${\rho\left( {t + T} \right)} = {{\frac{\lambda(t)}{\mu} \times \mu\; T} + {{\rho\left( {t + T} \right)} \times \left\lbrack {1 - {\mu\; T}} \right\rbrack\mspace{14mu}{DMOLModel}}}$

B. Rerouting and Call Gapping in the Peripheral Network Domain

FIG. 4 comprises, 4A, 4B, and 4C and is a flow diagram illustrating asecond embodiment of the invention in which network congestion isrelieved via the mechanisms of rerouting calls and call gapping in thePSTN domain of the peripheral networks that communicate with each othervia the central packet network. Rerouting and call gapping are separateand distinct congestion control mechanisms. However, the NCM merelygenerates the information needed to carry out the congestion reliefmechanism, while the actual mechanisms are carried out by the peripheralnetworks. Thus, as will become clear from the discussion below, thesteps of the NCM process in the central, packet, network are the samefor both of the peripheral domain congestion relief mechanisms(rerouting and call gapping). The difference in operation betweenre-routing and call gapping occurs in the peripheral domain (i.e., whatthe peripheral networks do with the provided information). They may beutilized entirely separately from each other or in conjunction with eachother and/or any of the other congestion control mechanisms discussedherein. For instance, the rerouting in the PSTN domain illustrated byFIG. 4 can be utilized as a secondary network congestion reliefmechanism if virtual trunk group resizing (FIG. 3) does not yield thenecessary congestion relief, e.g., does not bring the amount of callblocking, b_(i), below the call blocking threshold T. Rerouting and callgapping are illustrated in conjunction with each other in FIG. 4 simplybecause much of the processing for both mechanisms is similar. FIG. 4illustrates a level-1 embodiment in which the information is collectedand congestion relief mechanisms are identified for execution by othernetwork elements. The corresponding level-O embodiment should beapparent therefrom.

Generally, there would not be a corresponding level-2 embodiment (inwhich the NCM carries out the congestion control mechanisms) becausethese congestion control mechanisms are PSTN domain mechanisms whichmust be executed in the PSTN networks in any event.

Call gapping requires the ability to recognize the traffic sourcesfeeding into a VTG, e.g., the relevant PSTN switches. In practice, therealso may be multiple PSTN switches feeding into a given VTG. In thiscase, the VTG must be able to segregate the aggregated call attempt rateinto a VTG into its feeding components (i.e., the contributing PSTNswitches). Such functionality is more likely to be incorporated into acall processing gateway with SS7 call control capability than in a mediaprocessing gateway such as an AC120/60.

Referring now to FIG. 4, variables are defined in block 400. The inputsare the fraction of blocked calls per VTG (b), the call arrival rate perVTG (λ), the bandwidth per call, the number of channels per VTG (N), theblocking threshold (T), the average holding time per call (1/μ), thenetwork connectivity and topological information.

In step 401, variable i is set to 1. Variable i is used to designate theparticular VTG of the network being processed. In step 403, a VTGcorresponding to i is selected. In step 405, a variable M_(i) is set tothe size, N_(i), of the selected VTG. The size is designated in terms ofthe number of DSO channels in the VTG. In step 407, it is determinedwhether the fraction of blocked calls for VTG_(i) is less than thethreshold, i.e., if b_(i)<T. If it is, then no congestion managementactions are necessary with respect to the present VTG and, therefore iis incremented to select the next VTG (step 409) and flow returns tostep 403 to process the next VTG.

If, however, the call blocking being experienced exceeds the threshold,T, flow proceeds to step 411 where M is incremented by one. In steps 411and 413, channel capacity variable M_(i) is adjusted using equations 1a,1b, 2a, and 2b as described above in connection with FIG. 3 until thenecessary channel capacity to bring the call blocking rate, b_(i), belowthe call blocking threshold, T, is determined. In step 415, a valueD_(i) is determined which is the difference between the present size ofthe VTG and the size of that VTG needed to bring the call blockingprobability number below the predetermined call blocking threshold (ascalculated in steps 411–413).

In step 417, the VTG is inserted into an ordered list, L1, of VTGs thatare exceeding the call blocking threshold. The list is ordered such thatthe VTG corresponding to the highest value for D_(i) is listed first andthe VTG corresponding to the lowest value of D_(i) is listed last. Flowthen proceeds to step 419 to determine if all VTGs have been checked. Ifnot, i is incremented (step 409) and flow returns to step 403 forprocessing of the next VTG.

When all VTGs have been checked, flow proceeds from step 419 to decisionstep 421. In step 421, if list L1 is empty (i.e., if no VTGs are overlycongested), then the operation ends since no congestion reliefoperations are necessary. However, if list L1 is not empty, flowproceeds to step 423.

In step 423, the first VTG is removed from list L1 for processing. Instep 425, a peak cell rate for the desired size of that VTG iscalculated using equation (4). Then, in step 427, a list L2 is createdof gateway nodes (other than the source node of the current VTG) fromwhich there exists a path to the destination gateway node of the currentVTG. Next, decision step 429 determines if list L2 is empty. If so, thenflow returns to step 421 for processing of the next VTG on list L1, ifany.

If list L2 is not empty, i.e., if there are alternate source gatewaysfrom which calls from the relevant peripheral network can be routed tothe destination gateway node, then the first potential source gateway onlist L2 is removed from the list (step 431). In step 433, the path fromthat gateway to the destination gateway of the current VTG is examinedto determine its PCR using equation (4). In decision step 435, if thepath can accommodate a VTG having the PCR calculated in step 425, flowproceeds to step 437 where the necessary information, including theidentity of the alternate source gateway node, the identity of VTG_(i),the PCR of the new path and the fraction of calls from VTG_(i) thatshould be rerouted in the PSTN domain (D_(i)/M_(i)) is forwarded to thePSTN network for implementation. That PSTN network should then reroutethe fraction D_(i)/M_(i) of calls through the identified alternatesource gateway and/or institute call gapping based on the fractionD_(i)/M_(i). As discussed above, how this data is used by the peripheralnetwork will differ depending on whether it will perform call gapping orrerouting. For instance, if the peripheral network performs only callgapping, but no rerouting, then the alternate gateway information is notused by the peripheral network.

C. Voice Compression

FIG. 5 illustrates an ATM embodiment of the invention in which voicecompression is utilized as a congestion relief mechanism. As with eachof the previously discussed congestion relief mechanisms, this mechanismmay be used independently and separately from any of the others.Alternately, it may be used as just one of many relief mechanisms. FIG.5 comprises portions 5A and 5B and, for illustrative purposes, is alevel-0 embodiment in which the data is merely collected.

Variables are defined in block 500. The inputs are the fraction ofblocked calls per VTG (b), the call arrival rate per VTG (λ), the numberof channels per VTG (N), the blocking threshold (T), the average holdingtime per call (1/μ), the current compression rate (CCR), and networkconnectivity and topological information.

In step 501, variable i is set to 1. Variable i designates theparticular VTG of the network. In step 503, VTG_(i) is selected. In step505, variable M_(i) is set to the actual size, N_(i), of the selectedVTG. In step 507, it is determined whether the fraction of blocked callsfor VTG_(i) exceeds the threshold, i.e., if b_(i)>T. If the current VTGis experiencing less call blocking than the threshold level, then, insteps 508, 509, 511 and 515, a size, N_(i), is calculated for this VTGthat is the minimum size needed to keep the call blocking probabilityb_(i) below the blocking threshold, T. (These steps are essentiallyidentical to steps 308, 309, 311 and 315 from FIG. 3).

If the blocking rate exceeds the threshold, then flow proceeds from step507 to step 517. Steps 517–521 are similar to steps 508–515, except thatthe VTG size is being increased, rather than decreased. Specifically, instep 517, the non-stationary call blocking probability for a VTG of sizeM_(i) is calculated using equations 1a, 1b, 2a and 2b. In step 519, itis determined if the calculated probability is less than thepredetermined call blocking threshold. If not, then M is incremented byone (step 521) and steps 517 and 519 are repeated. The process flowsthrough steps 521, 517 and 519 until M_(i) yields a blocking rate thatis less than the blocking threshold T.

Regardless of whether the particular VTG under consideration is overlycongested (i.e., processed through steps 517–521) or not (i.e.,processed through steps 508–515), in step 523, a voice compression rateis calculated for each VTG based on the VTG size M_(i) determined insteps 508–515 of 517–521. The compression rate can be computed as:$\begin{matrix}{{{{compression}\mspace{14mu}{rate}} = {\frac{{CCRi} \times {Ni}}{Mi}\mspace{14mu}{Kbps}}},} & {{Eq}.\mspace{14mu}(5)}\end{matrix}$where CCR_(i) is the current compression rate, N_(i) is the actualpresent size of the VTG, and M_(i) is the ideal VTG size justdetermined.

In decision step 525, it is determined whether the compression ratecalculated in step 523 is equal to a standard compression rate. If so,that compression rate is assigned (step 527). If not, then the higheststandard compression rate that is less than the compression ratecalculated in step 523 is selected (step 529). In decision step 531, itis determined whether all VTGs have been processed. If not, i isincremented to select the next VTG (step 533) and flow returns to step503 to process the next VTG. If so, then the procedure ends.

In short, in the embodiment illustrated in FIG. 5, regardless of whetheror not a VTG is overly congested, it is assigned a compression rate, ifany, that is the lowest compression rate that will keep the callblocking probability below the predetermined call blocking threshold.Thus, for instance, as can be seen from the flow diagram, steps 508–515and 523–529 assure that, when traffic for a VTG decreases, the voicecompression ratio is reduced or eliminated accordingly. Likewise, astraffic increases, voice compression ratio is implemented (orincreased).

The compression rate will be applied only to calls initiated after it isdecided to use voice compression. The compression rate (or lack thereof)cannot be changed in the middle of a call. Accordingly, this solutionwill take some period of time to yield results.

The corresponding level-1 embodiment would include an additional step offorwarding the voice compression rate determined in step 527 or 529 tothe relevant source nodes for implementation. With respect to voicecompression as the congestion relief mechanism, it would not bepractical to develop a level-2 embodiment, since, in any event, thecompression would be implemented at the source gateway nodes.

In general, since all VTGs may not have the same quality of serviceobjective, this embodiment may be further modified in order to refinethe above described procedures on a per quality of service class basis.

D. Conclusions about ATM Networks

It is believed that one of the most effective combinations of the fourabove-discussed traffic congestion relief mechanisms for an ATM networkis a combination of all four mechanisms in the following hierarchy:

-   (1) virtual trunk group reconfiguration, (2) rerouting of calls in    the PSTN domain, (3) voice compression, (4) PSTN call gapping.

IV. IP Network Congestion Management Embodiments

Having considered several embodiments of the invention particularlyadapted for ATM packet networks, let us now turn to further potentialembodiments of the invention particularly adapted for IP networks. Asnoted above, IP networks generally have much less available congestioninformation. What we believe to be the most effective traffic controlmechanisms with respect to IP networks in order from most desirable toleast desirable are call blocking, voice compression, rerouting of callsin the PSTN domain and PSTN domain call gapping.

In the present specification, congestion control mechanisms for IPnetworks are classified into two classes. One class is independent ofthe real time protocol (RTP) measurements as set forth in “Real-TimeTransport Protocol Management InformationBase.”<draft-ietf-avt-etp-mib.txt>. The other class is based on RTPmeasurements in accordance with the RTP/RTCP protocol suite for datatransport and the ITU-T H.323v2 protocol suite for call signaling andcontrol. Embodiments using RTP measurements are preferable because ofthe availability of accurate packet loss and delay measurements.However, at the present time, RTP MIB modules are still in the draftstage and most Packet/Voice Gateways (PVGs) have not implemented themyet.

In IP networks, calls are typically admitted into the network withoutany admission control. Where admission control is applied, thecongestion control procedures proposed above for ATM networks would beapplicable.

In IP networks, when the network becomes congested, each call passingthrough the congested portion of the network starts to experience higherpacket losses and delays and, consequently, poorer quality. We shallconsider the goal to be to reduce the possibility that active callsexperience an unacceptable level of performance (via packet losses anddelays).

In networks utilizing RTP MIBs, call quality information is available interms of packet loss and delay jitter per call. However, even in theabsence of RTP MIBs, this information may be inferred with varyingdegrees of accuracy by collecting available measurements such as portutilization, buffer statistics and routing and topological information.After collecting the call quality information either through the packetloss and delay jitter per call data or as inferred through portutilization, buffer statistics and routing and topological information,the network control manager (NCM) in accordance with the presentinvention determines source-destination pairs (PVG pairs) whose callsare experiencing poor quality. The network administrator haspredetermined acceptable packet loss and delay jitter per callthresholds. When performance drops below those thresholds for anysource-destination pair, congestion control mechanisms are commenced.

In a preferred embodiment, the network congestion manager first tries toblock new call arrivals that can further deteriorate the quality of callperformance of existing calls. As a secondary measure, the NCM can applyhigher voice compression to new calls while maintaining the level ofvoice quality of existing calls so as to improve the quality of theexisting uncompressed calls and/or to allow more calls (less callblocking). Third, the NCM can try to reduce the number of active callsthrough that source-destination pair by having calls rerouted in thePSTN domain. Finally, if none of those measures produce adequateresults, the NCM can invoke call gapping functionality in the PSTNdomain.

Let us now consider a few particular embodiments of the inventionimplementing each of the above-discussed types of congestion controlmechanisms.

A. Call Blocking

FIGS. 6 and 7 are flow diagrams illustrating two embodiments utilizingcall blocking as the network congestion control mechanism. FIG. 6comprises portions 6A, 6B and 6C and illustrates an embodimentparticularly adapted for an IP network where RTP measurements areunavailable. FIG. 7 comprises portions 7A and 7B and illustrates anembodiment particularly adapted for IP networks in which RTPmeasurements are available. In these embodiments, the NCM determines,through the available measurements, what source-destination pairs arebeginning to experience poor performance (as defined by threshold set bythe network administrator). When these pairs are identified, the NCMinstructs blocking of a certain fraction of new arriving calls in orderto maintain the quality of existing calls.

Referring first to the RTP unaware embodiment of FIG. 6, variables aredefined in block 600. The input data are port utilizations (PU), routinginformation and topological information. In step 601, variable i, whichis used to represent the different ports of the network is set to 1. Instep 603, the i^(th) port is selected. In step 605, the set R_(i) of PVGpairs contributing traffic to the presently selected port is determinedfrom the available topological and routing information. In step 607, thenumber of calls, C, that can be accommodated at that port is determinedby multiplying the physical capacity of the port by a utilizationthreshold selected by the network administrator. The utilizationthreshold, U_(i), is a fraction (for instance, 0.8) of the physicalcapacity of the port that the administrator finds acceptable. In step609, the number of calls per second, α_(c), that can be accommodated atthat port based on the calculated value C (and other information) isdetermined by solving equation 1a for α_(c). In step 611, the calls persecond that are presently passing through that port, α_(A), isdetermined by summing the calls per second through each of the PVG pairscontributing to the traffic at that port.

In decision step 613, the under-utilized ports are segregated from theover-utilized ports. The under-utilized ports are processed throughsteps 615–623, while the over-utilized ports are processed through steps625–633. For purposes of the following discussion, it is important tobear in mind the distinction between ports and PVG pairs that contributetraffic to the ports.

The processing through the two routes is very similar in that abandwidth correction factor (either an incremental factor, p_(G), if thepresent utilization is less than the threshold, or a reduction factor,p_(B), if the present utilization is greater than the threshold) isdetermined for each PVG pair in set R for the present (i.e., the i^(th))port. Specifically, in step 615 (or 625), a bandwidth incremental (orreduction) factor is calculated for the i^(th) port. With respect to thereduction factor p_(B) (step 625), this value represents the fraction ofcalls through the ith port that need to be blocked to meet theutilization thresholds set by the administrator. With respect to theincremental factor p_(G) (step 615), this value represents the fractionof calls that can be added through the i^(th) port without inducing aneed to implement call blocking.

Then, in steps 627–633, a bandwidth reduction factor, f, for each PVGpair contributing traffic to the i^(th) port is assigned by taking thelargest of the reduction factors p_(B) of all of the overly congestedports to which each PVG pair in list R con-tributes. In other words, ifthe traffic between a given PVG pair passes through m ports, it isassigned a bandwidth reduction factor of max (p_(B,1), p_(B,2), p_(B,3),p_(B,4), . . . p_(B,m)). Likewise, in steps 617–623, a bandwidthincremental factor, e, for each PVG pair contributing traffic to thei^(th) port is assigned by taking the smallest of the incrementalfactors p_(G) of all of the under utilized ports to which each PVG paircontributes. In other words, if the traffic between a given PVG pairpasses through m under utilized ports, it is assigned a bandwidthincremental factor of min(p_(G,1), p_(G,2), p_(G,3), p_(G,4), . . .p_(G,m)).

Steps 617–623 and 627–633 are included because any given PVG pair may beprocessed through steps 613–633 more than once because steps 613–633will be traversed for every port in the network and any given PVG pairmay be contributing traffic to more than one port in the network.

After all of the PVG pairs contributing traffic to port i have beenprocessed, flow proceeds to step 635, where it is determined if all ofthe ports have been processed. If not, flow proceeds to step 637, wherei is incremented, and the next port is processed in accordance withsteps 603–633.

When all ports have been so processed, flow instead proceeds from step635 to step 639, where variable k is set to one. Variable k is used torepresent PVG pairs.

Another result of the fact that any given PVG pair could be contributingtraffic to more than one port is that it is possible that the same PVGpair might have been assigned a value for both e and f in steps 621 and631, respectively. It also is possible that any given PVG pair may nothave been contributing traffic to any of the ports. Thus, in step 641,it is first determined if the PVG pair was assigned either an e or a fvalue. If not, processing flows directly to decision step 653 where theprocedure either ends (if all PVG pairs have been processed) or k isincremented (step 655) and flow returns to step 641 for processing ofthe next PVG pair.

However, if, in step 641, either a f or e value was assigned, flowproceeds to decision step 643, where it is first inquired whether thisparticular PVG pair was assigned a bandwidth reduction value, f, i.e.,if it is over utilized. If so, then, in step 645, a new, decreased, callcapacity value, M_(k), is assigned for that pair by multiplying thenumber of calls for which bandwidth is allocated to that pair, N_(k), bythe bandwidth reduction factor f assigned to that PVG pair in step 631.Any bandwidth incremental factor, e, that may have been assigned to thatPVG pair (in step 621) is ignored if a bandwidth reduction factor, f,was assigned to that pair since, if any port to which that PVG pair wascontributing traffic was over utilized, call blocking should beincreased, not decreased, for that PVG pair.

If, on the other hand, no bandwidth reduction factor, f, was assigned tothat PVG pair in connection with any port, then, in step 647, it isdetermined if a bandwidth incremental factor, e, was assigned. Step 647is provided for illustrative purposes. However, in reality, it is asuperfluous processing step since, in order for flow to reach step 647,the condition expressed in step 647 already must be true. Thus, flowwill proceed to step 649. In step 649, a new, increased, call capacity,M_(k), is assigned for that pair by multiplying the number of calls forwhich bandwidth is allocated to that pair by the bandwidth incrementalfactor, e, determined for that pair.

In either event, flow proceeds to step 651, where a new call blockingprobability is calculated and assigned by using the newly calculatedcall capacity, M_(k), determined in step 645 or 649 in equation 1a.

In step 653, if all PVG pairs have been checked, the procedure ends.Otherwise, variable k is incremented (step 655) and flow returns to step641 for processing of the next PVG pair.

FIG. 6 illustrates a level-0 implementation. Level-1 and level-2implementations would include further steps for implementing theindicated call blocking rates, either directly or through other networkelements.

FIG. 7 is a flowchart comprising portions 7A and 7B and illustratingprocessing for call blocking in an IP network in which RTP measurementsare available. Variables are defined in block 700. The inputs in thisembodiment are the fraction of packet loss per call (L_(c)), the packetloss threshold established by the network administrator (L_(t)), thedelay jitter per call (D_(c)), the delay jitter per call thresholdestablished by the network administrator (D_(t)) and the PVG paircorresponding to each call.

In step 701, variables S and i are reset. Variable i represents aparticular active call, variable S represents a set of PVG pairs betweenwhich there are calls experiencing delay jitter and/or packet loss inexcess of the thresholds set by the administrator (to be explainedfurther below). Variable i is reset to 1 and set S is reset to null. Instep 703, the ith call is selected. In step 705, it is determinedwhether the call is experiencing inadequate performance by comparing itspacket loss and delay jitter statistics (as maintained in the RTPprotocol) with the performance thresholds set by the systemadministrator. If either threshold is not being met, then flow proceedsto step 707, where the PVG pair is added to set S. If both performancespecifications are being met, then flow skips step 707 and proceedsdirectly to step 709, where it is determined if all of the calls havebeen evaluated. If not, then i is incremented (step 711) and flowproceeds through steps 701–709 again for the next active call.

When all active calls for all PVG pairs in the network have beenevaluated, flow proceeds from step 711 to step 713. In step 713, avariable Q is defined as the set of PVG pairs in the network whose callsare experiencing acceptable performance (L_(C) and L_(T) are below thepredetermined thresholds). In step 715, each PVG pair identified asexperiencing acceptable performance, has a flag set that indicates thatnew calls can be admitted.

In step 717, if no PVG pairs have been added to set S in step 707, theprogram terminates since all calls are experiencing adequateperformance. If there are any calls in set S, then flow instead proceedsto step 719, in which a PVG pair in set S is selected. In step 721, anaverage packet loss across all calls using the particular PVG pair,A_(LC), is determined. In step 723, an average delay jitter across allcalls using the particular PVG pair, A_(LD), is determined. In step 725,it is determined if both the average delay jitter and the average packetloss for the PVG pair are within the corresponding, predetermined,thresholds. If so, then new calls are admitted (step 727). If not, thennew calls are not admitted (step 729). Flow then proceeds back to step717 for processing of the next PVG pair in set S until there are no PVGpairs remaining is set S.

Call Blocking Implementation Considerations

Neither RTP nor H.323 currently supports the concept of a virtual trunkgroup for an IP network. However, there is enough management informationin MIBs operating under the RTP or H.323 standard to logically define avirtual trunk group. This logic definition would require correlation ofMIB data that is maintained in different devices, such as source anddestination PVGs. A network congestion manager could be provided inaccordance with the present invention that would perform the correlationof this data and define virtual trunk groups for IP networks. In thismanner, the reconfiguration of virtual trunk groups described above withrespect to ATM networks could also be implemented with respect to IPnetworks.

Since call rejection decisions are made after the fact, that is, afterthe packet loss exceeds the packet loss threshold, the systemadministrator should set the packet loss threshold below the actualdesired packet loss objective. Further, it is advisable to collect callblocking statistics for purposes of long term traffic engineering, e.g.,determining what resources are necessary for the network and how theyshould be allocated.

Rerouting and Call Gapping in the PSTN Domain

FIG. 8 includes portions 8A, 8B, 8C and 8D and comprises a flow diagramillustrating both rerouting and call gapping in the PSTN domain. As withall other congestion control mechanisms discussed heretofore, reroutingin the PSTN domain and call gapping in the PSTN domain also can be usedindividually or in combination with each other or any of the othercongestion control mechanisms discussed in this specification. However,since much of the processing for both mechanisms is the same, bothmechanisms are incorporated in this figure.

As discussed above in connection with FIG. 4, a single flow chart can beused to illustrate rerouting and call gapping because the congestionrelief is performed in the peripheral domain. Thus, the differences inprocessing are in the peripheral domain and, therefore, do not manifestthemselves in the NCM flowchart of FIG. 8, which illustrates only theprocessing in the central, packet-based, network.

Also, there is little useful information with respect to the callgapping and rerouting provided in the RTP protocol. Accordingly, thereare no significant differences between RTP aware and RTP unawareembodiments. Thus, FIG. 8 generically applies to both RPT aware and RPTunaware embodiments of the invention.

Variables are defined in block 800. In step 801, variable i, whichrepresents a port of the network, is set to 1. In step 803, the i^(th)port is selected. In decision step 805, the under-utilized ports aresegregated from the over-utilized ports. In a RTP aware embodiment, thisdecision would be based on delay jitter and packet loss statistics asdiscussed above in connection with step 705 in FIG. 7. In a RTP unawareembodiment, it would be based on indirect indicators of congestion suchas port utilizations, buffer statistics and routing and topologicalinformation. If the present port is under-utilized, nothing is done withrespect to it and i is simply incremented (step 807) and flow returns tostep 803 for processing of the next port. If the port, however, is beingover utilized, then flow proceeds to step 809, in which the set R of PVGpairs contributing traffic to the port is determined from the availabletopological and routing information. In step 811, the number of calls,C, that can be accommodated at that port is determined by multiplyingthe capacity of the port by a utilization threshold selected by thenetwork administrator. The utilization threshold, U, is a fraction, forinstance, 0.8, of the total physical capacity of that port that theadministrator finds acceptable.

In step 813, the number of calls per second, α_(c), that can beaccommodated at that port based on the calculated value C is determinedby solving equation 1a for α_(c). In step 815, the actual calls persecond that are presently passing through that port, α_(A), isdetermined by summing the calls per second through each of the PVG pairscontributing to the traffic at that port.

In steps 817, a preliminary bandwidth reduction correction factor,p_(B), due to the i^(th) port is determined. This value is thefractional difference between the actual call rate being experienced andthe desired maximum call rate for the port. The larger this value, themore congested the port. Then, in steps 819, through 823 and 825, forevery PVG pair contributing traffic to port i, the bandwidth correctionfactor calculated for port i in step 817 is assigned to it, if and onlyif it is greater than any bandwidth correction factor previouslyassigned to that PVG pair.

By way of explanation, any given PVG pair may be contributing traffic toany port. Therefore, any PVG pair could have been assigned a bandwidthcorrection factor on a previous pass through steps 817–823 in connectionwith another port to which it is contributing traffic. Steps 817–823guarantee that the highest bandwidth correction factor of any port towhich any PVG pair is contributing traffic is assigned to that PVG pair.

When it is determined in step 821 that all PVG pairs in set R_(i) (i.e.,PVG pairs contributing traffic to the ith port) have been checked, i.e.,j=m, flow proceeds to decision step 827. If there are ports that stillremain to be checked, i is incremented (step 807) and flow returns tostep 803 for processing of the next port.

When all of the ports have been checked, flow instead proceeds from step827 to step 829, where k is set to 1. Variable k is used to representthe PVG pairs in the network.

In decision step 831, it is determined if a bandwidth correction factorhas been assigned to the k^(th) PVG pair. If so, flow proceeds to step833 where an ideal channel capacity for the k^(th) PVG pair isdetermined by multiplying the bandwidth correction factor, f, by theactual channel capacity, N_(i), of the ith PVG pair.

In step 835, variable M_(i) is set to N_(i). In steps 837 and 839, thevariable M_(i) is adjusted using equations 1a, 1b, 2a, and/or 2b asdescribed above in connection with FIG. 3 until the channel capacitynecessary to bring the call blocking rate below the call blockingthreshold, T, is determined. In step 841, a value D_(i) is set to thedifference between the channel capacity N_(i) of the k^(th) PVG pair asset in step 833 and the channel capacity value, M_(k), determined insteps 837 and 839 necessary to bring the call blocking rate, b_(i),below the call blocking threshold, T.

In step 843, the PVG pair is inserted into an ordered list, L1, of PVGpairs that are exceeding the call blocking threshold. The list isordered such that the PVG pair corresponding to the highest value forD_(i) (i.e., the most congested PVG pair) is listed first and the PVGpair corresponding to the lowest value of D_(i) is listed last. Flowthen proceeds to step 845 to determine if all PVG pair have beenchecked. If not, i is incremented (step 847) and flow returns to step831 for processing of the next PVG pair.

When all PVG pairs have been checked, flow proceeds from step 845 todecision step 849. If list L1 is empty (i.e., if no PVG pairs remain inthe list L1), then the operation ends since no further congestion reliefoperations are necessary. However, if list L1 is not empty, flowproceeds to step 851.

In step 851, the first PVG pair is removed from list L1 for processing.In step 853, a packet-rate, PPR, for the number of calls which it isdesired to reroute (D_(k)) is calculated. Then, in step 855, a list L2of gateway nodes, other than the source node of the current PVG pair,from which there exists a path to the destination gateway node of thecurrent PVG pair is created from the network routing and topologicalinformation. Next, decision step 857 determines if list L2 is empty. Ifso, then flow returns to step 849 for processing of the next PVG pair inlist L1, if any. If list L2 is not empty, i.e., if there are potentialalternate source gateways from which calls from the peripheral networkcan be routed to the destination gateway node, then the first potentialsource gateway in list L2 is removed from the list (step 859). In step861, the shortest path from that gateway to the destination gateway ofthe PVG pair under investigation is identified from the network routingand topological information. In decision step 863, it is determined ifall the ports through which that path traverses are within theutilization threshold. If so, flow proceeds to step 865 where thenecessary information, including the identity of the alternate sourcegateway node, the packet rate (PPR) which it is desired to reroute, andthe fraction of calls between that PVG pair that should be rerouted (orto which call gapping should be applied) in order to bring the trafficthrough the PVG pair under consideration within the predeterminedthreshold (D_(k)/M_(k)) is forwarded to the PSTN network forimplementation. The PSTN network should then reroute the fractionD_(k)/M_(k) of calls through the identified alternate source gatewayand/or institute call gapping based on D_(i)/M_(i) and PPR_(i).

In short, in the embodiment of FIG. 8, first, the congested ports areidentified (steps 801–809). Then, the amount of traffic beingcontributed to that port by each PVG pair in the system is determinedand the fractional amount of traffic contributed to that port by eachPVG pair is quantified (steps 809–825). Next, the amount of trafficbetween each PVG pair which it is desired to reroute is determined(steps 829–843). Finally, the alternate gateways through which trafficmay be routed is determined (steps 849–863).

Voice Compression Congestion Control

FIG. 9, comprising portions 9A, 9B, 9C and 9D, is a flow diagramillustrating the use of voice compression as the congestion controlmechanism in an IP network. This diagram illustrates a level-1embodiment. The voice compression processing may be used independentlyfor congestion relief or in conjunction with any of the other congestionrelief mechanisms discussed herein. As was the case with respect to callgapping and rerouting in the peripheral domain as discussed above inconnection with FIG. 8, the RTP protocol does not provide significantinformation with respect to voice compression. Accordingly, the steps ofthe process are essentially the same whether or not the embodiment isRTP aware. Thus, FIG. 9 is generic to both RTP aware and RTP unawareembodiments.

Variables are defined in block 900. In step 901, variable i, whichrepresents a port of the network is set to 1. In step 903, the i^(th)port is selected. In step 905, the set R_(i) of PVG pairs contributingtraffic to the presently selected port (the i^(th) port) is determinedfrom the available topological and routing information. In step 907, thenumber of calls, C, that can be accommodated at that port is determinedby multiplying the capacity of the port by a utilization thresholdselected by the network administrator. The utilization threshold, U_(i),is a fraction, for instance, 0.8, of the total physical capacity of thatport that the administrator finds acceptable. In step 909, the number ofcalls per second, α_(c), that can be accommodated at that port based onthe calculated value C is determined by solving equation 1 a for α_(c).In step 911, the calls per second that are presently passing throughthat port, α_(A), is determined by summing the calls per second througheach of the PVG pairs contributing to the traffic at that port.

In decision step 913, the under-utilized ports are segregated from theover-utilized ports. As discussed above in connection with FIG. 8, thecriteria used to make the decision depends on whether the network is RTPaware or unaware. If RTP aware, then delay jitter per call and packetloss per call data is compared to predetermined thresholds. Otherwise,more indirect evidence of congestion is used. The under-utilized portsare processed through steps 915–923, while the over-utilized ports areprocessed through steps 925–933. The processing through the two routesis very similar in that a bandwidth correction factor (either anincremental factor, P_(G), if the present utilization is less than thethreshold, or a reduction factor, p_(B), if the present utilization isgreater than the threshold) is determined for each PVG pair in set R.Specifically, in step 915 (or 925), a bandwidth reduction (orincremental) factor is calculated for the i^(th) port. With respect tothe reduction factor p_(B) (step 925), this value represents thefraction of calls through the i^(th) port that would need to be blockedin the absence of voice compression. With respect to the incrementalfactor p_(G) (step 915), this value represents the fraction of callsthat can be added through the i^(th) port without inducing a need toimplement call blocking in the absence of voice compression.

Then, in steps 927–933, the bandwidth reduction factor, f, for each PVGpair contributing traffic to the i^(th) port is assigned by taking thelargest of the reduction factors p_(B) of all of the overly congestedports to which each PVG pair contributes traffic. In other words, if thetraffic between a given PVG pair passes through m ports, it is assigneda bandwidth reduction factor of max (p_(B,1), p_(B,2), p_(B,3), p_(B,4),. . . p_(B,m)). Likewise, in steps 917–923, a bandwidth incrementalfactor, e, for each PVG pair contributing traffic to the i^(th) port isassigned by taking the smallest of the incremental factors p_(G) of allof the under utilized ports to which each PVG pair contributes. In otherwords, if the traffic between a given PVG pair passes through m underutilized ports, it is assigned a bandwidth incremental factor of min(p_(G,1), p_(G,2), p_(G,3), p_(G,4), . . . p_(G,m)).

Steps 917–923 and 927–933 are included because any given PVG pair may beprocessed through steps 913–933 more than once because steps 913–933will be traversed for every port in the network and any given PVG pairmay be contributing traffic to more than one port in the network.

After all of the PVG pairs contributing traffic to port i have beenprocessed through steps 913–933, flow proceeds to step 935, where it isdetermined if all of the ports have been processed. If not, flowproceeds to step 937, where i is incremented, and the next port isprocessed in accordance with steps 903–933.

When all ports have been so processed, flow instead proceeds from step935 to step 939, where variable k is set to one. Variable k representsPVG pairs.

Another result of the fact that any given PVG pair could be contributingtraffic to more than one port is that it is possible that the same PVGmight have been assigned both a value for p_(B) and a value for p_(G) insteps 921 and 931, respectively. It also is possible that any given PVGpair may not have been contributing traffic to any of the ports and,therefore, not been assigned any p_(B) or p_(G). Thus, in step 941, itis first determined if the PVG pair was assigned either a p_(B) or ap_(G) value. If not, processing flows directly to decision step 959where the procedure either ends if all PVG pairs have been processed, ork is incremented (step 961) and flow returns to step 941 for processingof the next PVG pair.

However, if, in step 941, either a p_(B) or p_(G) value was assigned,flow proceeds to decision step 943, where it is inquired whether thisparticular PVG pair was assigned a bandwidth reduction factor, f. If so,then, in step 945, a new, decreased, call capacity value, M_(i), isassigned for that pair by multiplying the number of calls for whichbandwidth is allocated to that pair, N_(k), by the bandwidth reductionfactor, f, assigned to that PVG pair in step 931. Any bandwidthincremental factor, e, that may have been assigned to that PVG pair isignored if a bandwidth reduction factor, f, was assigned to that pairsince, if any port to which that PVG pair was contributing traffic wasover utilized, voice compression for that PVG pair should be increasedregardless of traffic through other ports. If, on the other hand, nobandwidth reduction factor was assigned to that PVG pair, then it isdetermined if a bandwidth incremental factor, e, was assigned (step947). Like step 647 in FIG. 6, step 947 is for illustrative purposesonly since, if flow reaches step 947, the condition set forth thereinmust have been true. Hence, flow will proceed to step 949. In step 949,a new, decreased, call capacity, M_(k), is assigned for that pair bymultiplying the number of calls for which bandwidth is allocated to thatpair by the bandwidth incremental factor e for that pair.

In either event, flow proceeds to step 951, where a new voicecompression ratio is determined using the newly calculated callcapacity, M_(k), determined in step 945 or 949 that would maintain thecall blocking probability below the predetermined threshold, T. Notethat, if step 951 was reached through step 945, then the compressionratio will be increased in step 951 to relieve congestion. If, on theother hand, step 951 was reached through step 949, then the compressionratio will be decreased in step 951 since no port to which that PVG pairis contributing traffic is experiencing poor quality of service and thusthe compression ratio being used, if any, may be decreased. In step 952,N_(k) is set to M_(k).

From step 9542, flow proceeds to steps 953–957 where a new compressionratio is actually assigned to the PVG pair. Specifically, if thecompression ratio c determined in step 951 is equal to a standardcompression ratio, then that ratio is assigned to that PVG pair (step955). If not, then the highest of the standard compression ratios thatis less than c is assigned to that PVG pair (step 957). Flow thenproceeds to step 959 for processing of the remaining PVG pairs, if any.

In accordance with the invention, a plurality of methods and apparatusfor detecting congestion and implementing congestion relief mechanismshas been presented.

Having thus described a few particular embodiments of the invention,various alterations, modifications, and improvements will readily occurto those skilled in the art. Such alterations, modifications andimprovements as are made obvious by this disclosure are intended to bepart of this description though not expressly stated herein, and areintended to be within the spirit and scope of the invention.Accordingly, the foregoing description is by way of example only, andnot limiting. The invention is limited only as defined in the followingclaims and equivalents thereto.

1. A method of relieving congestion in a packet-based network thatinterconnects a plurality of peripheral networks by implementingcorrective actions in said plurality of peripheral networks, saidperipheral networks each comprising a plurality of switches, and saidpacket-based network comprising a plurality of gateway nodes having datato be transferred therebetween, and utilizing a concept of virtualpipelines between said gateway nodes, said pipelines comprising one ormore channels, said method comprising the steps of: (1) identifying afirst set of virtual pipelines for which traffic exceeds a predeterminedthreshold; (2) for each virtual pipeline in said first set of virtualpipelines, determining a number of additional channels needed to causesaid traffic through said pipeline to not exceed said predeterminedthreshold; and (3) for each pipeline in said first set of virtualpipelines, assigning a corrective action and an amount of saidcorrective action to be taken in said peripheral networks as a functionof said number of additional channels, wherein said corrective actioncomprises assigning a call gapping rate for each switch in saidplurality of peripheral networks contributing traffic to said pipeline,and wherein said call gapping rates are(λt−λ_(N))/λ_(t) where λ_(t) is a call arrival rate for a correspondingpipeline; and λ_(N) is a call arrival rate corresponding to saidpredetermined threshold for said corresponding pipeline.
 2. The methodset forth in claim 1 wherein said predetermined threshold is a callblocking ratio and wherein step (2) comprises determining a minimumpipeline size that would reduce the call blocking ratio for saidpipeline below said predetermined threshold based on a call arrival rateat said virtual pipeline and an average holding time per call.
 3. Themethod set forth in claim 2 wherein said minimum pipeline size isexpressed as a number of channels, M, in said pipeline and wherein step(2) comprises determining a number of channels M by;${B_{M}\left( {\lambda_{i}/\mu} \right)} = {\frac{\left( {\lambda_{i}/\mu} \right)^{M}/{M!}}{\sum\limits_{n = 0}^{M}\;{\left( {\lambda_{i}/\mu} \right)^{n}/{n!}}}\mspace{14mu}{wherein}}$ρ(t) = λ((t)/μ(t)  orρ^(′)(t) = λ(t) − ρ(t)/μ(t)
 4. The method set forthin claim 3 wherein ρ(t)=λ((t)iμ(t) is used when call arrival ratethrough said pipeline has been historically increasing andρ′(t)=λ(t)−ρ(t)μ(t) is used when call arrival rate through said pipelinehas been historically decreasing.
 5. The method set forth in claim 2wherein said network is an asynchronous transfer mode network.
 6. Themethod set forth in claim 5 wherein said network is used to exchangevoice data.
 7. The method set forth in claim 6 wherein said othernetworks comprises time division multiplexed networks.
 8. A method ofrelieving congestion in a packet-based network that interconnects aplurality of peripheral networks by implementing corrective actions insaid peripheral networks, said peripheral networks each comprising aplurality of switches, and said packet-based network comprising aplurality of gateway nodes having data to be transferred therebetween,and utilizing a concept of virtual pipelines between said gateway nodes,said pipelines comprising one or more channels, said method comprisingthe steps of: (1) identifying a first set of virtual pipelines for whichtraffic exceeds a predetermined call blocking ratio threshold; (2) foreach virtual pipeline in said first set of virtual pipelines,determining a number of additional channels needed to cause said trafficthrough said pipeline to not exceed said predetermined threshold; and(3) for each pipeline in said first set of virtual pipelines, assigninga corrective action and an amount of said corrective action to be takenin said peripheral networks as a function of said number of additionalchannels wherein said corrective action comprises rerouting calls insaid peripheral networks that would so that they pass through adifferent pipeline in said packet-based network; wherein step (2)comprises determining a minimum pipeline size that would reduce the callblocking ratio for said pipeline below said predetermined thresholdbased on call arrival rate at said virtual pipeline and average holdingtime per call.
 9. The method set forth in claim 8 wherein said minimumpipeline size is expressed as a number of channels, M, in said pipelineand wherein step (2) comprises determining a number of channels M by;${B_{M}\left( {\lambda_{i}/\mu} \right)} = {\frac{\left( {\lambda_{i}/\mu} \right)^{M}/{M!}}{\sum\limits_{n = 0}^{M}\;{\left( {\lambda_{i}/\mu} \right)^{n}/{n!}}}\mspace{14mu}{wherein}}$ρ(t) = λ((t)/μ(t)  orρ^(′)(t) = λ(t) − ρ(t)μ(t).
 10. The method setforth in claim 8 wherein said network comprises a plurality of gatewaynodes through which calls in said peripheral networks enter and exitsaid packet-based network and wherein step (3) comprises the steps of:(3.1) determining a peak cell rate corresponding to said number ofadditional channels; (3.2) determining if there is at least one gatewaynode corresponding to a source peripheral network for which there existsa path in said packet-based network to a destination node of saidpipeline; (3.3) determining if said path can accommodate a pipelinehaving said peak cell rate; and (3.4) generating data indicating one ormore alternate source gateways.
 11. The method set forth in claim 10further comprising the step of: (3.5) outputting said data to saidperipheral networks.
 12. The method set forth in claim 10 furthercomprising the step of: (4) organizing prior to step (3) said pipelinesidentified in step (1) in order of said number of additional channelsdetermined in step (2); and wherein steps (3.1), (3.2), (3.3), (3.4) and(3.5) are performed for each pipeline identified in step (1) in orderfrom said pipeline corresponding to said highest number of additionalchannels to said lowest number of additional channels.
 13. The methodset forth in claim 12 wherein step (3.2) comprises creating a list ofall gateway nodes and wherein step (3.3) is performed with respect toeach source gateway in said list of gateways until a source gatewayhaving a path to said destination gateway that can accommodate apipeline having said peak cell rate is located.
 14. An apparatus forrelieving congestion in a packet-based network that interconnects aplurality of peripheral networks by implementing corrective actions insaid network, said packet-based network comprising a plurality ofgateway nodes having data to be transferred therebetween, and utilizinga concept of virtual pipelines between gateway nodes of said network,said pipelines comprising one or more channels, said method comprisingthe steps of: means for identifying a first set of virtual pipelines forwhich traffic exceeds a predetermined threshold; means for determining,for each virtual pipeline in said first set of virtual pipelines, anumber of additional channels needed to cause said traffic through saidpipeline to not exceed said predetermined threshold; and means forassigning, for each pipeline said first set of virtual pipelines, acorrective action and an amount of said corrective action to be taken insaid peripheral networks as a function of said number of additionalchannels, wherein said corrective action comprises assigning a callgapping rate for each peripheral network contributing traffic to saidpipeline and wherein said call gapping rates are$\frac{\lambda_{i} - \lambda_{N}}{\lambda_{i}}$ where λ_(t) is a callarrival rate for a corresponding pipeline; and λ_(N) is a call arrivalrate corresponding to said predetermined threshold for saidcorresponding pipeline.
 15. The apparatus set forth in claim 14 whereinsaid predetermined threshold is a call blocking ratio and wherein saidmeans for determining comprises determining a minimum pipeline size thatwould reduce the call blocking ratio for said pipeline below saidpredetermined threshold based on a call arrival rate at said virtualpipeline and an average holding time per call.
 16. The apparatus setforth in claim 15 wherein said minimum pipeline size is expressed as anumber of channels, M, in said pipeline and wherein step (2) comprisesdetermining a number of channels M by;${B_{M}\left( {\lambda_{i}/\mu} \right)} = {\frac{\left( {\lambda_{i}/\mu} \right)^{M}/{M!}}{\sum\limits_{n = 0}^{M}\;{\left( {\lambda_{i}/\mu} \right)^{n}/{n!}}}\mspace{14mu}{wherein}}$ρ(t) = λ((t)/μ(t)  orρ^(′)(t) = λ(t) − ρ(t)μ(t).
 17. The apparatus setforth in claim 16 wherein ρ(t)=λ((t)/μ(t) is used when arrival call ratethrough said pipeline has been historically increasing andρ′(t)=λ(t)−ρ(t)μ(t) is used when call arrival rate through said pipelinehas been historically decreasing.
 18. The apparatus set forth in claim15 wherein said network is an asynchronous transfer mode network. 19.The apparatus set forth in claim 18 wherein said network is used toexchange voice data.
 20. The apparatus set forth in claim 19 whereinsaid other networks comprises time division multiplexed networks.
 21. Anapparatus for relieving congestion in a packet-based network thatinterconnects a plurality of peripheral networks by implementingcorrective actions in said network said packet-based network comprisinga plurality of gateway nodes having data to be transferred therebetweenand utilizing a concept of virtual pipelines between gateway nodes ofsaid network, said pipelines comprising one or more channels, saidmethod comprising the steps of: means for identifying a first set ofvirtual pipelines for which traffic exceeds a predetermined threshold;means for determining, for each virtual pipeline in said first set ofvirtual pipelines, a number of additional channels needed to cause saidtraffic through said pipeline to not exceed said predeterminedthreshold; and means for assigning, for each pipeline in said first set,a corrective action and an amount of said corrective action to be takenin said peripheral networks as a function of said number of additionalchannels, wherein said corrective action comprises rerouting calls insaid peripheral networks so that they pass through a different pipelinein said packet-based network.
 22. The apparatus set forth in claim 21wherein said predetermined threshold is a call blocking ratio andwherein said means for determining comprises determining a minimumpipeline size that would reduce the call blocking ratio for saidpipeline below said predetermined threshold based on call arrival rateat said virtual pipeline and average holding time per call.
 23. Theapparatus set forth in claim 22 wherein said minimum pipeline size isexpressed as a number of channels, M, in said pipeline and wherein step(2) comprises determining a number of channels M by;${B_{M}\left( {\lambda_{i}/\mu} \right)} = {\frac{\left( {\lambda_{i}/\mu} \right)^{M}/{M!}}{\sum\limits_{n = 0}^{M}\;{\left( {\lambda_{i}/\mu} \right)^{n}/{n!}}}\mspace{14mu}{wherein}}$ρ(t) = λ((t)/μ(t)  orρ^(′)(t) = λ(t) − ρ(t)μ(t).
 24. The apparatus setforth in claim 22 wherein said network comprises a plurality of gatewaynodes through which calls in said peripheral networks enter and exitsaid packet-based network and wherein said means for assigningcomprises: means for determining a peak cell rate corresponding to saidnumber of additional channels; means for determining if there is atleast one gateway node corresponding to a source peripheral network forwhich there exists a path in said packet-based network to a destinationnode of said pipeline; means for determining if said path canaccommodate a pipeline having said peak cell rate; and means forgenerating data indicating one or more alternate source gateways. 25.The apparatus set forth in claim 24 further comprising: means foroutputting said data to said peripheral networks.
 26. The apparatus setforth in claim 24 further comprising: means for organizing saidpipelines identified by said means for identifying said first set ofvirtual pipelines in order of said number of additional channelsdetermined by said means for identifying; and wherein said means forassigning acts on each pipeline identified by said means for identifyingsaid first set of virtual pipelines in order from said pipelinecorresponding to said highest number of additional channels to saidlowest number of additional channels.
 27. The apparatus set forth inclaim 26 wherein said means for determining if there is at least saidone gateway node comprises means for creating a list of all gatewaynodes and wherein said means for determining if said path canaccommodate a pipeline having said peak cell rate acts upon each sourcegateway in said list of gateways until a source gateway having a path tosaid destination gateway that can accommodate a pipeline having saidpeak cell rate is located.